Oxygen getter for high pressure sodium vapor lamp



Dec. 23, 1969 P. J. JORGENSEN OXYGEN GETTER FOR HIGH PRESSURE SODIUM VAPOR LAMP Filed Aug 28, 1967 inventor: Pd lddo$en8fg is Attorney United States Patent U.S. Cl. 206.4 Claims ABSTRACT OF THE DISCLOSURE In a high pressure sodium vapor lamp in which the arc tube is a highly transparent alumina ceramic, the elemental sodium can react with the small amounts of oxygen or water which are present and in turn react with the alumina of the arc tube, resulting in the removal of free sodium from the interior of the arc tube. The use of a metal composed of yttrium, for example, has inhibited this reaction to the extent that the electrical characteristics of the lamp remain constant for periods of operation of the order of about 2500 hours. It has been found that this period of time can be substantially improved by doping the metal with from about 2 to 20 percent by weight of a cation metal having a plus four valence such as a 90% yttrium, thorium alloy, for example.

CROSS-REFERENCE TO RELATED APPLICATION Application Ser. No. 753,143, filed Aug.116, 1968, by Rodney E. Hanneman, Paul J. Jorgensen and Richard J. Charles, entitled, High-Intensity Vapor Arc-Lamp," a continuation-in-part of application Ser. No. 616,538, filed Feb. 16, 1967, in the names of the same inventors, now abandoned, same assignee.

BACKGROUND OF INVENTION The invention relates to high-pressure sodium vapor lamps utilizing alumina ceramic for the arc tube and it is particularly concerned with the prevention of sodium cleanup, that is reduction in the elfective amount of sodium during the life of the lamp.

The high pressure sodium vapor lamp with which the invention is more particularly concerned generally comprises an outer vitreous envelope or jacket within which is mounted a slender tubular arc tube of high density polycrystalline alumina. The basic lamp type is described and claimed in U.S. Patent No. 3,248,590, issued Apr. 26, 1966, to Kurt Schmidt. The are tube encloses a charge of sodium, preferably a filling of sodium-mercury amalgam, and an inert gas such as xenon. It operates with sodium vapor pressure sufiicient to cause appreciable broadening and self-reversal of the resonance lines, the range from 30 to 500 torr being preferred. A large percentage of the total radiation is emitted on either side of the yellow resonance D lines of sodium at 5890 and 5896 angstroms resulting in a golden white light having a relatively large amount of energy in the red. The xenon is a starter gas for the lamp and the mercury is a buffer gas producing the proper temperature distribution in the plasma and at the envelope walls. The presence of the mercury increases the voltage gradient of the arc, resulting in a lamp operating at a higher voltage and lower current for a given wattage and this makes for a more efficient lamp and permits saving in ballast costs. Even though the partial pressure of mercury in the lamp may be several times greater than that of sodium, little radiation of mercury lines is apparent in the visible spectrum and it is essentially only the sodium atoms that are excited to produce light.

In the development of this lamp, it was found early that over a prolonged period of use, the voltage drop across the lamp increased and the proportion of red in 'ice the radiation decreased, the light becoming more purple; the latter phenomenon is generally referred to as decrease in the red factor. At the same time the mercury vapor pressure increased and, if operation were continued notwithstanding the deterioration in light output, the rising mercury vapor pressure would cause the voltage drop across the lamp to increase to the point where the lamp extinguished. It was determined that these happenings were associated with a sodium loss and a reduction in the proportion or atom fraction of sodium in the excess liquid sodium-mercury amalgam present in the reservoir during operation. Such loss or cleanup of sodium could be reduced by lowering the amalgam reservoir temperature; however, this also lowers both lamp efficacy and the red factor and is at best only a compromise. The solution for this problem which was set forth in the previously crossreferenced patent application was the provision of a metal in communication with the interior of the arc tube which was composed of a reactive material such as, for example, yttrium, cerium or lutecium, yttrium being the preferred material.

The principal object of the present invention is to provide a better and more thorough solution to the problem of sodium cleanup in lamps of the present kind by the provision of a more efficient longer lived reactant.

SUMMARY OF THE INVENTION When yttrium was employed as a reactant material as disclosed in the previously referenced patent application, the removal of free sodium from the interior of the alumina ceramic arc tube was reduced to the extent that the electrical characteristics of the lamp remained constant for about 2500 hours of operation, however, the lamp voltage began to rise thereafter indicating that the yttrium was becoming less effective.

It was postulated that a protective layer of yttrium oxide (Y O was being formed at the surface of the yttrium which was acting as a barrier to the passage of oxygen and preventing oxygen diffusion. into the yttrium. It was then proposed that if the yttrium oxide was doped with a cation having a plus four valence, that oxygen diffusion could be accomplished interstitially and the theoretical life of the material more closely approached. It was found that such an addition significantly improves the oxygen diffusion kinetics of the reactive material.

Those skilled in the art will gain a better understanding of this invention from the following detailed description taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevational view of a high pressure sodium vapor lamp with parts broken away for purposes of illustration; and,

FIG. 2 is an enlarged fragmentary detail view of one suitable form of the upper end cap assembly of a discharge tube such as shown in FIG. 1 which includes the improved reactant.

As shown in FIG. 1, a high pressure sodium vapor lamp 1 comprises an outer vitreous envelope or jacket 2 and a conventional threaded base 3. The inner envelope or are tube 4 is made of sintered, transparent, high density, polycrystalline alumina ceramic. The upper end of the arc discharge tube is provided with an end cap 5 hermetically sealed to the upper end of tube 4 and provided with appropriate electrical connections and support structure indicated generally at 6.

In FIG. 2, the details of the upper end of tube 4 and end cap assembly 5 are shown in detail. End cap assembly 5 consists of member 7 made of niobium and hermetically sealed to are tube 4 at 8. Tungsten electrode 9 is bonded to niobium shank 10 which is of tubular form and provides a capsule-like portion 11 which is welded or brazed to the neck portion 12 of member 7 providing a hermetic seal there between. The interior of portion 11 provides a receptacle for the reactant 13. The upper end 15 of portion 11 is crimped and welded shut after the reactant is placed Within the capsule portion. The niobium end cap structures are permeable to oxygen at lamp temperature operating conditions so that any oxygen present in either the interior of the arc tube or in the inter-envelope space may diffuse through the niobium and react with the contained reactant.

The improved reactant is composed of from about 2 to 20 percent by weight of a cation having a plus four valence, balance yttrium. Preferred combinations are yttrium-thorium, yttrium-hafnium, and yttrium-zirconium alloys. An alloy composed of 90 percent yttrium and 10 percent thorium has been found to be particularly efiicacious as a reactant in this environment. It will be appreciated that mixtures of these dopants may also be used.

The details of construction of the preferred embodiment which have been illustrated and described are intended as exemplary and not in order to limit the invention thereto except insofar as included in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An article of manufacture comprising a hollow capsule composed of a metallic material having a high permeability to oxygen at elevated temperatures, said capsule providing a container for a reactive alloy, and a reactive alloy substantially filling said capsule consisting essentially of from 2 to 20 percent by Weight of a cation metal having a plus four valence selected from the group of thorium, hafnium, zirconium and mixtures thereof, balance substantially all yttrium.

2. The article set forth in claim 1 wherein said capsule consists'essentially of niobium.

3. Thearticle set forth in claim 1 wherein said reactive alloy'consists, essentially of thorium and yttrium.

4. The article set forth in claim 1 wherein said reactive alloy consists essentially of hafnium and yttrium.

5. The article set forth in claim 1 wherein said reactive W alloy consists essentially of zirconium and yttrium.

References Cited UNITED STATES PATENTS 3,384,798 5/1968 Schmidt 313184 JAMES W. LAWRENCE, Primary Examiner PALMER C. DEMEO, Assistant Examiner US. Cl. X.R. 

